The major goal of this project is to understand how potassium ions are transported by epithelia. Two major epithelia which are responsible for the regulation of plasma potassium levels are the kidney and the colon. I propose to study the colon as a model system for investigating K+ transport mechanism. Specifically the route of K+ movement across the epithelium will be examined and active and passive transport systems that are responsible for maintenance of intracellular K+ composition will be characterized. The electrical features of the epithelium will be studies using a number of converging methods. Membrane and junctional resistances will be resolved using D.C. microelectrode methods and impedance analysis. Membrane permeabilities and driving forces will be determined from intracellular ion activity measurements using liquid ion exchanger microelectrodes and radioisotopic flux methods. In addition electrical methods will be used to assess the properties of colon crypts. This information will be incorporated into an equivalent circuit model of the epithelium and will be used to construct a comprehensive model for K+ transport across the colon.